1999 Civic EX engine: D16Y8 Automatic Transmision 135K miles
It all started while driving at highway speeds the car would start stumbling.
Gradually it got harder and harder to start the car when cold.
No problem restarting it though once warm.
What's been changed: fuel filter, spark plugs, cap/rotor, wires.
By priming (turning the key to on without engaging the starter) until you hear the main relay click off and repeating this priming for 15 times or more, the engine starts without much of a problem.
Also keep in mind that the engine bogs down when the accelerator is applied heavily. When you floor it, the engine seems to want to stall.
What can be wrong?
Possible fuel pump filter bag clogged up,
fuel pump pressure (bad pump?),
fuel pressure regulator.

 

check fuel pressure.
check your tps as well, do a voltage sweep on it.
i was gonna say check your egr valve but your egine dont have one.

 

For me , with these information , I would check my catalyser first, block cat has these problem , after that i'll check the fuel system with freeze data and etc ....

Let us know and sorry for my english (im a french one )

 

Throw some seafoam/injector cleaner in the tank is the first thing I'd try just to see if it helps but it definitely sounds like it isn't getting enough fuel (TPS and even blocked cat make some sense too but priming several times probably wouldn't help in that case) Maybe the pump is on it's last legs or a pretty bad blockage somewhere - regardless if you haven't ran some sort of injector/fuel system cleaner in a long while I'd try that first. (If you recently did use an injector/fuel system cleaner and shortly after had the issues, it's a blockage, probably in the tank at the filter bag - rare but it can happen with some products in extreme circumstances, if you stick with the more common products there's no reason to worry)